Apparatus for recording and reading an image on a medium and detecting errors and media defects

ABSTRACT

Apparatus is disclosed for recording and reading a color image formed on a receiving medium such as a film coated with a blue metastable colloidal silver. Upon application of heat to selected areas of the coating, the blue silver turns yellow to form an image. In a record mode, a modulated beam from a diode laser is scanned onto the receiving medium to form an image. In a read mode, a diode laser beam containing two different wavelengths of radiation is scanned across an image on the recording medium. In order to detect errors in the image, a detector senses the two wavelengths at each pixel and produces signals indicative of the densities of the yellow and blue in the pixel. Signal processing means is adapted to determine valid data points from the two signals.

Reference is made to U.S. patent application, Ser. No. 07/634,621,entitled "A Method for Recording and Reading An Image," filed in thenames of Gilmour et al. on even date herewith; this application isassigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application relates to an apparatus for recording and reading animage, and more particularly, to such apparatus which is adapted torecord and read a multicolor image.

2. Description of the Prior Art

In optical digital data recording, the recording medium can be a majorcontributor to signal errors. Missing or spurious signals induced bymedia defects cause data loss or contamination of the recordedinformation. Consequently, error detection and correction (EDAC)capability is incorporated into recording and playback systems. Suchcapability is usually in the form of software programs to encode anddecode signals. During the recording phase, redundancy is built in bysignal coding. During playback, the signal is verified or corrected by adeciphering code.

The proportion of a recording medium which is dedicated to errorcorrection can be significant; for example, thirty percent is notunusual. Higher redundancy is required if the medium is imperfect, or ifa very low error content is needed. Bit error rates of one in 10¹³ orless are sought in some applications. With present technology, opticalmedia can not be manufactured to that degree of perfection.

The cost of EDAC is high not only in terms of the data storage densitybut also in terms of the data processing time and the complexity of therecording and decoding devices. Clearly, a system which can discriminatebetween recorded data and media defects offers an advantage. Thebenefits of such a system would include lower bit error rates and lessredundancy which would result in higher data storage density.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the problemsdiscussed above in the prior art and to provide improved apparatus forthe recording and reading of an image.

In accordance with one aspect of the invention, there is providedapparatus for reading an image on an image-bearing medium, the imagebeing formed from a plurality of colors, each pixel in the imagecontaining the colors in a predetermined relationship, the apparatuscomprising: a source of radiation for directing radiation containing aplurality of wavelengths onto the medium; detector means for sensingeach of the wavelengths at each pixel of the image and for providingsignals representative of the densities of the colors in the pixel; andsignal processing means for processing the signals from the detectormeans, the signal processing means including means for determining therelationship of the colors in each of the pixels and for providing anoutput indicative of whether the information in the pixel is valid.

In one embodiment of the present invention, a recording medium in theform of a web is supported for movement in a page scan, or cross-scan,direction. The medium can be, for example, a film having a bluecolloidal silver coating. A yellow image is formed on the blue coating,and there is a predetermined relationship between the blue and theyellow in each pixel of the image. In a record mode, a galvanometerscans a laser beam across the recording medium in a scan direction. Adiode laser, which is driven in accordance with an information signal,supplies the laser beam to the galvanometer through a collimator lensand beam shaping optics.

In a read mode, an unmodulated beam of radiation from the diode laser isdivided into two different wavelengths of radiation and thedual-wavelength beam is directed to the galvanometer. The galvanometerscans the beam across a recording medium having a color image recordedthereon. After the beam passes through the recording medium, the beam isintercepted by a dual-wavelength detector which provides signalsrepresentative of the densities of radiation in the two wavelengths. Thesignals are directed to signal-processing means which produces a signalindicative of whether each picture element or pixel in the image isvalid information.

A principal advantage of the present invention is that errors in arecording medium can be detected without using large areas of therecording medium for error correction. A further advantage is that errordetection in an optical recording system can be performed withoutelaborate software schemes for coding and decoding the data. The presentinvention is particularly suitable for producing an optical recording ofbinary data which is relatively error free.

Other features and advantages will become apparent upon reference to thefollowing description of the preferred embodiment when read in light ofthe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of the apparatus of the presentinvention;

FIG. 2 is a schematic representation showing elements of the apparatusof the present invention which are used in the read mode;

FIG. 3 is a schematic drawing of a signal processing circuit for use inthe present invention;

FIG. 4 is a schematic drawing of a signal processing circuit forproducing a signal indicative of whether a defect is opaque ortransparent;

FIG. 5 is a schematic drawing of a signal processing circuit whichcombines the features of the circuits shown in FIGS. 3 and 4; and

FIG. 6 is a graph showing the signals produced by a two-color mediumwhen read with a two-wavelength source of radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, there is shown apparatus 10 constructed inaccordance with the present invention. Apparatus 10 comprises a sourceof radiation 14 which produces a beam of radiation 16 that is modulatedin accordance with an information signal. Beam 16 is directed to agalvanometer 20 through a collimating lens 22, beam shaping optics 24,and a turning mirror 23. Beam shaping optics 24 includes a pair ofcylindrical lenses 25 and 26. Galvanometer 20 is adapted to scan thebeam onto a receiving medium 30. The beam 16 from galvanometer 20 passesthrough an f-θ lens 32 which functions in a well-known manner tomaintain a flat field and a constant velocity of the scanned spot on therecording medium.

Apparatus 10 is adapted to operate in a record, or write, mode in whichinformation is recorded on medium 30. As shown in FIG. 1, receivingmedium 30 extends in the form of a web between a supply roll 42 and atake-up roll 44. Rolls 42 and 44 can be driven, for example, by astepper motor (not shown) which is actuated in timed relation to thescan movement of galvanometer 20 to advance the medium 30 in across-scan direction. As will be explained hereinafter, apparatus 10 canalso be operated in a read mode to check for errors in the informationrecorded on medium 30 and/or electronically record the image on themedium. A multiwavelength detector 40, located behind the medium, isused in the read mode.

Radiation source 14 can include a laser 43, for example, a diode laserwhich emits radiation at 830 nm. One suitable laser is a ModelSDL-24200H2 laser manufactured by Spectrodiode Laboratories. Source 14also includes a driver 44 for laser 43 and a frequency doubler device 46which can be selectively moved into the path of the output beam fromdiode laser 43. Frequency doubler device 46 is used in the read mode forerror detection in a manner to be explained below. Device 46 includes abeam splitter 48 which passes one portion of the beam to a beamequalizer 50 and another portion of the beam to a turning mirror 52.From turning mirror 52, the beam portion passes through a frequencydoubler element 54 which produces an output beam at one-half thewavelength of the input beam. The output beam is directed by means of aturning mirror 58 to a dichroic mirror 56 which serves to recombine thetwo beam portions to form beam 16. The beam 16, which in the read modefor error detection contains wavelengths of 830 nm and 415 nm, is thendirected to the galvanometer 20.

The frequency doubler element 54 can be a Model KDP, obtainable fromCleveland Crystal Co., or a lithium niobate device obtainable from thesame company. The beam equalizer 50 can be a circular wedge neutraldensity filter, No. 03FDC003, obtainable from Melles Griot Co. It willbe apparent to those skilled in the art that the multiwavelength beamcould be produced in other ways, e.g., by combining the beams from twoseparate sources of radiation or by using a broad-band light source incombination with wavelength selective filters.

The recording medium in the present invention can be any medium whichrecords information in color, for example, films containingthermochromic compounds and mediums in which a color coupler is covertedinto a color image dye to record a signal. A preferred recording medium,however, is a film coated with a metastable silver coating, for example,as disclosed in U.S. application, Ser. No. 493,026, entitled "Method ofThermally Forming Images From Metastable Metal Colloids," filed on Mar.13, 1990 in the names of Gilmour and Shuman; this application isassigned to the assignee of the present invention, and the disclosure inthe application is expressly incorporated herein by reference. Asdisclosed in the Gilmour and Shuman application, a thermal image can beformed on the medium having a blue metastable colloidal silver coatingby providing sufficient heat to significantly raise the temperature ofthe silver layer. In the areas where heat is applied to the bluecolloidal silver, the blue is changed to yellow. Such heat can beprovided, for example, by a short duration pulse from a diode laser. Thebackground color of the metastable silver need not be blue. Any of awide variety of colors, including orange and magenta, can be achieved byhalting the amplification process employed in forming the metastablesilver at an early stage.

As noted above, apparatus 10 can be operated in a read mode. In one readmode, beam 16 would be scanned across medium 30 in the manner describedabove, and light transmitted through the medium would be sensed toelectrically record the image on the medium.

In another read mode, recording errors in the medium 30 can be checked.In this mode, the frequency doubler 46 would be inserted in the opticalpath, and the beam containing the two wavelengths would be scanned ontothe receiving medium 30. Detector 40 would produce signals representingthe densities of the two wavelengths to signal processing circuitry 60.

A picture element, or pixel, exposed on the blue colloidal silver willhave blue areas and yellow areas. When combinations of blue and yellowfractional areas of a pixel are read, there will be a single-valuedrelation between the blue and yellow signals. That is, for any yellowvalue, there will be a corresponding blue value. Further, an increase inone value will be accompanied by a decrease in the other; thisrelationship is shown graphically in FIG. 6 where the blue value (theblue transmission signal at 415 nm) is represented by the solid line andthe yellow value (red transmission signal produced by infrared radiationat 830 nm) is represented by the dotted lines. Any departure from thisrelationship will indicate the presence of a defect, and it is thischaracteristic which is used in the present invention to detect errorsin information recorded on the medium 30.

In the case of binary images or binary recorded data, a departure fromthe expected relationship of blue and yellow values can be used todetect errors caused by imperfections in the medium. For example, anopaque area, such as a dirty area, will cause a decrease in both red andblue transmission signals. Conversely, a transparent area, such as ascratch in the film, will cause an increase in both signals. Thereforeboth of these defects would be detectable. Sufficiently large defectscause both signals to vary in the same direction instead of in opposingdirections. Thus, both the presence and nature of a medium defect can bedetected. A knowledge of the nature of the defect can enhance EDACefficiency, thereby saving space on the recording medium as well asreducing the read/write time.

In the case of multilevel images or recorded data, two estimates ofpixel exposure can be obtained: one from the blue transmission signaland one from the red transmission signal. The combination of bothsignals, properly weighted for signal to noise, gives a more accurateestimate than either value alone.

The image of a continuous tone object, whether rendered in continuous orhalf tone, can be read as a negative or a positive. Reading the"negative" (blue transmission signal decreasing with exposure) with bluelight gives the most information in the low-exposure region where theblue density is least. Reading the "positive" (red transmission signalincreasing with exposure), the S/N (signal-to-noise ratio) is highest atthe high exposures where the red density is least. In fact, the twosignals can be combined statistically to yield more information thaneither signal alone.

Signals can be processed from detector 40 in accordance with thecircuitry 60 shown in FIG. 3. As shown in FIG. 3, a signal from an IRdetector (not shown) in detector 40 would be passed to a pulse gate andbinary converter 62, and from pulse gate and binary converter 62, asignal would be fed to AND gate 64. A signal from a blue detector (notshown) in detector 40 would pass through a signal inverter 66, a pulsegate and binary converter 68, and then to AND gate 64. A HIGH outputfrom AND gate 64 would indicate a valid data point. The pulse gate andbinary converters 62 and 68 can each be a Model No. NE527, made bySignetics Co. The AND gate 64 can be a No. SN74ALS00, obtainable fromTexas Instruments Co.

In FIG. 4, there is shown a circuit 70 for determining whether a defectis the result of a scratch or dirt. In circuit 70, a signal from theblue detector is passed to a pulse gate and binary converter 72 and toanother pulse gate and binary converter 74 through a signal inverter 76.Similarly, a signal from the IR detector is passed to a pluse gate andbinary converter 78 and to another pulse gate and binary converter 80through a signal inverter 82. Signals from pulse gate and binaryconverters 72 and 78 are passed to AND gate 84. A signal from AND gate84 is fed to OR gate 86. A HIGH output from AND gate 84 would indicate ascratch, since both the blue and IR signals would be high. Signals frompulse gate and binary converters 74 and 80 are fed to AND gate 88, and aHigh output from AND gate 88 would indicate dirt, since signals fromboth the blue and IR detectors would be low. A HIGH output from OR gate86 would indicate that either a scratch or dirt was present on medium30.

A circuit 90 in FIG. 5 functions in the same manner as circuit 70 toindicate when either a scratch or dirt is present. In addition an ANDgate 92 receives signals produced in the manner described above forcircuit 60 and produces a HIGH output when a valid pixel is present.

This invention has been described in detail with particular reference tothe preferred embodiment thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. For example, although the apparatus has been describedherein as operating in a transmission mode, it will be apparent to thoseskilled in the art that the apparatus could operate in a reflectionmode.

We claim:
 1. Apparatus for reading an image on an image-bearing medium,said image being formed from a plurality of colors, each pixel in theimage containing the colors in a predetermined relationship, saidapparatus comprising:a source of radiation for directing radiationcontaining a plurality of wavelengths onto said medium; detector meansfor sensing each of said wavelengths at each pixel of said image and forproviding signals representative of the densities of the colors in thepixel; and signal processing means for processing the signals from saiddetector means, said signal processing means including means fordetermining the relationship of said colors in each of said pixels andfor providing an output indicative of whether the information in saidpixel is valid and whether the medium at the pixel location is free ofdefects.
 2. Apparatus, as defined in claim 1, wherein said medium is afilm coated with blue metastable colloidal silver coating.
 3. Apparatus,as defined in claim 1, wherein said source of radiation includes a laserwhich produces a beam of radiation at one wavelength.
 4. Apparatus, asdefined in claim 3, wherein said source includes means for producingmultiple wavelengths in said beam of radiation.
 5. Apparatus, as definedin claim 4, wherein said source includes means for producing wavelengthsof 830 nm and 415 nm in said beam of radiation.
 6. Apparatus, as definedin claim 1, wherein said detector means includes means for detecting twowavelengths of said plurality of wavelengths in said beam and forproducing signals representative of the densities of said twowavelengths in each of said pixels.
 7. Apparatus, as defined in claim 6,wherein said detector means includes means for combining the signalsrepresentative of said two wavelengths to produce said output indicativeof whether the information in the pixel is valid.
 8. Apparatus, asdefined in claim 1, wherein said signal processing means includes meansfor indicating the presence of an opaque area on said medium, saidopaque area being an area of low radiation transmission thatsubstantially blocks radiation transmission through said medium. 9.Apparatus, as defined in claim 1, wherein said signal-processing meansincludes means for indicating the presence of a transparent area on saidmedium, said transparent area being an area substantially void of saidplurality of colors.
 10. Apparatus for recording and reading an image ona recording medium, said image being formed from a plurality of colors,each pixel in the image containing the colors in a predeterminedrelationship, said apparatus comprising:a source of radiation, saidsource being adapted to selectively produce radiation on said medium ina single wavelength or in a plurality of wavelengths; means formodulating said radiation in accordance with an information signal toproduce said image; means for directing radiation from said source ontosaid medium; detector means for sensing each of said wavelengths at eachpixel of said image and for providing signals representative of thedensities of the colors in the pixel; and signal processing means forprocessing the signals from said detector means, said signal processingmeans including means for determining whether said colors are in saidpredetermined relationship at each of said pixels and whether there isan opaque area that substantially blocks transmission of radiationthrough said medium or a transparent area void of said plurality ofcolors.